1. Field of the Invention
The present invention is related to containers of the type having at least one heat-sealed juncture region, and more particularly, to such containers formed from foam polymer sheet materials.
Foamed polystyrene is used for products such as egg cartons, food containers, meat trays, etc., since this material is readily thermoformable. However, many container structures are not suitable for thermal formation. Such structures include envelopes, multi-sided cartons, pouches for items such as french fries and others. These structures require a material which can be folded without cracking or breaking at the fold point. This excludes conventional foam polystyrene and poly(p-methylstyrene). Also, for these structures it is necessary or practical that juncture points of the panels of the structures be fastened together by heat sealing. However, foamed polystyrene generally has not been satisfactory for heat sealing, perhaps because of the thickness differential between the actual sealed portion and the remaining foamed structure.
2. Description of the Prior Art
U.S. Pat. No. 4,201,818 to Rohn describes a flexible plastic foam formed by partially crushing a foam sheet between textured rollers. The treatment is said to provide a product suitable as a replacement for paper. The disclosure of this patent is incorporated herein by reference. UK patent application No. 2,096,616A includes a similar disclosure.
U.S. Pat. No. 3,256,527 to Studen describes a process for forming envelopes by heat sealing. Two layers of foam are heat-sealed through the application of heat and pressure. Heat is also supplied for a short distance beyond the area in which pressure is applied.